1. Field
The present disclosure relates to a method for controlling a fuel cell system.
2. Description of the Related Art
A solid polymer electrolyte fuel cell includes a membrane electrode assembly (MEA), in which an anode electrode is disposed on one surface of an electrolyte membrane, which is a proton-conductive polymer ion-exchange membrane, and a cathode electrode is disposed on the other surface of the electrolyte membrane. The membrane electrode assembly and separators, sandwiching the membrane electrode assembly, constitute a power generation cell (unit cell). Typically, a predetermined number of power generation cells are stacked and, for example, mounted in a fuel cell vehicle (a fuel cell electric automobile or the like) as a vehicle fuel cell stack.
The fuel cell generates electric power by causing an electrochemical reaction between a fuel gas (such as hydrogen gas) supplied to the anode electrode and an oxidant gas (such as compressed air) supplied to the cathode electrode. During power generation, protons are generated at the anode electrode, and the protons are conducted through the electrolyte membrane and move to the cathode electrode. At the cathode electrode, protons, electrons, and oxygen in the oxidant gas react to generate water.
The electrolyte membrane needs to be moist so that the electrolyte membrane can conduct protons. For this purpose, a humidifier is disposed in an oxidant gas supply channel as described, for example, in Japanese Unexamined Patent Application Publication No. 2005-190843. The oxidant gas is moistened while passing through the humidifier, and the moist oxidant gas and the generated water keep the electrolyte membrane moist. A bypass channel, which bypasses the humidifier, branches off from the oxidant gas supply channel.
However, for example, when the output electric current of the fuel cell is small, the electrochemical reaction is not accelerated, and therefore the amount of generated water is small. Accordingly, the electrolyte membrane becomes drier. In this state, the impedance of the fuel cell increases. Conversely, the impedance decreases when a sufficient amount of moisture is added to the electrolyte membrane.
Therefore, some fuel cell systems have a mechanism for determining whether the electrolyte membrane is becoming dry or becoming moist from a measured impedance value. For example, Japanese Unexamined Patent Application Publication No. 7-235324 describes that whether or not the cathode electrode is excessively moist is determined on the basis of the impedance of the fuel cell and that the oxidant gas is supplied through the bypass channel when it is determined that the cathode electrode is excessively moist.